The present invention relates to a memory system having a semiconductor memory device and, more particularly, to a memory system which protects principal data stored in the semiconductor memory device from being copied.
In a television game system for example, the system has usually a microcomputer having an erasable programmable read only memory device (EPROM) which is a type of a programmable read only memory device (PROM) which is electrically programmable. The game can be performed by the microcomputer, controlled by the program data stored in the EPROM. Since the program involves the software of the television game, it is very important to protect the program data from being copied. Generally, creating a program is a hard task and requires a lot of man-hours, so that user access to the program data causes great economic damage to a system maker and a program owner.
Not only in the television game system, but also in other systems, a program is so important that the system maker and program owner must take precautions against dishonest persons who try to imitate same and sell a similar system by copying the program data. Therefore, it has been required to achieve a memory means from which the stored data cannot be copied.
Efforts for protecting the stored data from the memory device have been made, but only on the simple substance of a memory device. For example, there is a semiconductor memory device improved so that the data stored in the memory device cannot be read from the simple substance of the memory device. However, since the measure is taken only in the memory device, it can be easily overcome by the persons skilled in the art. Then, there are many illegal copies in the market. Thus, protection against improper retrieval of stored data from a memory device presents a big problem in the field of the semiconductor memory device.